rsync flags: -a, -n, -p, --exclude, --delete

Feature changes across 15 files:

- -a/--archive: enables -c -m -M (no -s, which slows transfers)
- -n/--dry-run: scan + print without connecting or transferring
- -p <port>: custom SSH port (passed as -p to ssh via execvp)
- --exclude <pattern>: glob-based filename filtering in scanner
- --delete: sender collects file manifest; receiver deletes unlisted files

Protocol: added STATUS_MANIFEST, use_delete field in Config wire format.
New utilities: glob_match(), delete_extras() with recursive directory walk.
Scanner: accepts exclude patterns; skips matching entries.
SSH transport: switched from execlp to execvp for dynamic port arg.
Server + multiprocessing: handle STATUS_MANIFEST in both single and
multithreaded receive paths.

Builds clean, all 7 tests pass.

src/client/scanner.h

@@ -12,9 +12,11 @@ typedef struct {
   char *current_path;
   bool use_metadata;
   unsigned long long chunk_size;
+  char **exclude_patterns;
+  int exclude_count;
 } DirectoryScanner;
 
-DirectoryScanner *directory_scanner_create(char *root_directory, bool use_metadata, unsigned long long chunk_size);
+DirectoryScanner *directory_scanner_create(char *root_directory, bool use_metadata, unsigned long long chunk_size, char **exclude_patterns, int exclude_count);
 Chunk *directory_scanner_next(DirectoryScanner *scanner);
 void directory_scanner_destroy(DirectoryScanner *scanner);